I'm fairly sure the iPhone YouTube app has been displaying all the same videos since launch day. "Featured" and "Most Viewed" have barely changed from what I can see.
YouTube isn't about watching the most popular videos (though many people do). It's about broadcasting the off-the-wall stuff that you wouldn't normally watch or can't get through mainstream media.
is there a search function in iPhone's youtube.app? what will happen if you just navigate to youtube.com and click a video inside iSafari?